The previous two chapters have looked at scenarios for making deep reductions in GHG emissions in the transportation sector by 2050. We now turn to considering what role the transportation sector might play under economy-wide CO2 constraints in the United States. If we see emission reductions achieved in different sectors of the economy—including commercial and residential buildings, industry, agriculture, and electric power, as well as transportation—as wedges that add up to an emission reduction target mandated by policy, how might the transportation wedge reduce its emissions to meet the policy goals under optimized least-cost solutions? Will economy-wide carbon taxes and cap-and-trade programs result in emission reductions from the transportation sector commensurate with its contribution to economy-wide emissions? Or are other approaches needed to incentivize the transportation climate mitigation wedge? To address these questions, we used an integrated energy-economics model called the MARKet ALlocation (MARKAL) model to examine least-cost emission reductions scenarios within economy-wide emission cap scenarios.